1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly to an image forming apparatus using a stepping motor for a drive mechanism.
2. Description of Related Art
In image forming apparatuses such as copiers, printers, and multifunction peripherals (MFPs), a stepping motor is used as the drive source, in some cases. In such image forming apparatuses, a constant-current drive system (hereinafter referred to as a “constant-current system”) is employed as a system for controlling a stepping motor in many instances. In the constant-current system, at the start of driving, a relatively high voltage is applied to a stepping motor until a target current flows. When the current flowing in the stepping motor attains the target current, then pulse width modulation (PWM) control is performed to maintain the target current. That is, in the constant-current system, after the current flowing in the stepping motor attains the target current, the current is continuously monitored, and control to maintain the target current is performed. For the control to maintain the target current, a switching system of rapidly repeating on/off operations for applying voltage is employed.
Regarding image forming apparatuses as mentioned above, various techniques have been hitherto disclosed.
For example, Japanese Laid-Open Patent Publication No. 07-067016 discloses a technique that, in an image processing apparatus, corrects the rotary torque of a motor into an optimum state using the rotation speed and the rotation direction of the motor as parameters.
Japanese Laid-Open Patent Publication No. 11-180003 discloses a technique that, in an image forming apparatus, detects change in the rising of a drive current for a motor and counts the number of chopping of the drive current to detect the conditions of rotation of the motor, and controls the drive current based on the detected conditions.
Note that, in image forming apparatuses, various sensors are provided in addition to such motors to detect paper jam in a paper feed mechanism, the presence or absence of an ink cartridge attached in an image forming unit, and so forth.
As the aforementioned target current, a current is set which is anticipated in the case where the load on a portion with which the stepping motor is used is maximum. This is intended to ensure operations without causing step out of the motor even when the load on the above portion is maximum. Note that the step out refers to a phenomenon in which when the cycle of pulse signals of a drive voltage provided to a stepping motor becomes too short, operations of the rotor of the stepping motor cannot follow such a cycle, with the result that the rotor does not rotate normally.
The setting of the constant-current value mentioned above is specifically described with reference to the drawing.
FIG. 24 is a graph showing the relationship between changes over time of the load torque required for driving a stepping motor and the constant-current value to be set. With reference to FIG. 24, the load torque increases with time as indicated by the line A. The load on a portion with which the stepping motor is used is maximized at the initial stage in use, where continuous printing operations is carried out for a predetermined time, and in specific environments or a combination of these conditions. In the conventional constant-current system, to ensure operations of the motor without causing the step out of the motor even when an image forming apparatus is in such conditions as described above and the load torque is maximized, or even when a change occurs suddenly to temporarily increase the load torque, the constant-current value in accordance with the maximum load torque, as indicated by the line B, is set.
When the image forming apparatus is not in the conditions as described above, that is, in most of the times during which the image forming apparatus is driven, the load is not maximum as shown in FIG. 24. During such a time, the stepping motor operates with the current setting that causes excessive motor output with respect to the load torque required.
If the motor output is excessive with respect to the load, surplus energy is produced in addition to the required energy. The surplus energy increases the vibration of the stepping motor. As a result, there arises a problem that the noise generated when the image forming apparatus is driven becomes large.
The motor output is excessive in most of the times during which the image forming apparatus is driven, and therefore there is also a problem that energy is excessively consumed in the image forming apparatus.
Further, heat generation in the stepping motor and the drive power source increase because of generation of surplus energy, and therefore there is also a problem that a cooling mechanism is sometimes needed.
Further, in an image forming apparatus as described above, reduction in manufacturing costs is thought much of, just as for generally-used devices, and there is a demand for making the image forming apparatus have as few components as possible to promote downsizing.